The insulators concerned have a glass dielectric, are of the cap-and-pin type and form suspension strings intended in particular for medium, high and very high voltage power transmission lines (for example 150 kV and 250 kV lines).
When a leakage current flows through a string of insulators, the latter are subjected to arcing activity. These arcs generally begin, i.e. are initiated, at the metallic parts, in the transition zone between a conductor and an insulator, in other words, at the base of the insulator cap.
Heretofore, the caps have been protected against corrosion by galvanizing their outside suface. But the arcs flashing from the metal parts tend to erode this protective coating. The caps then become directly exposed to agents of atmospheric corrosion, which eventually degrade the mechanical properties of the metal parts and so contaminate the surface of the insulator by the iron oxide produced by corrosion.
It is the object of the present invention to obviate this disadvantage without modifying any of the following insulator characteristics:
their commercial power frequency operation in both dry and wet conditions; PA1 their lightning withstand capability, PA1 and their radio interference level.